The invention relates to a method for supplying powder to a coating tank for electrophoretic powder coating and an apparatus for excuting the method.
Recently, an electrophoretic powder coating method, which is generally called an EPC method, has been developed and has remarkably improved the coating technique. The EPC method is the electrophoretic coating of cationic type in which a combination of cataphoresis, electrolysis, electric separatinon and electroosmose is used for forming a coating layer on the surface of an article, as in the conventional anionic type electrophoretic coating method. One of the differences of the EPC method from the conventional anionic type electrophoretic coating method is to use coating material in which insoluble powder is suspended and dispersed in diluted solution of water-soluble binder resin (cationic binder). Another difference is that a DC voltage is applied between an article to be coated and an electrode immersed in the plating bath in order that the article serves as a cathode and the electrode an anode. For this, the coating layer formed on the surface of the article is composed of cationic binder resin and the powder. The EPC method offers many advantages in the following. (a) A thick coating layer may be formed for a short time, e.g. for several seconds. (b) The thickness of the coating is adjustable at will. (c) No elution of the metal from the article is occurs, because the coating is excuted by means of cationic type electrophoretic coating. (d) The coating layer formed is excellent in tightness and corrosion resistance. (e) The coating work is hygienic one wiht no harm by solvent and without possibility of explosion by dust. (f) Powder may be collected approximately at 100%.
In the EPC method, the powder contained in the plating bath is gradually consumed as the coating work progresses, with the result that the ratio of the amounts between the powder and binder in plating bath changes. When the ratio falls below a predetermined value, the coating quality deteriorates. Therefore, it is necessary to supply powder of the amount corresponding to the consumed powder amount to the plating bath, in mid course of the coating work. In this case, however, the powder supplied gathers into lumps so that it is hard to disperse into the plating bath. Even if it is dispersed, aging of the powder and binder is insufficient in the plating bath. Many attempts to solve such problems have been made but unsuccessful.